1. Field of the Invention
The present invention pertains to the art of cooking appliances and, more particularly, to a microwave feed system for a toroidal waveguide in a microwave cooking appliance.
2. Discussion of the Prior Art
Cooking appliances utilizing a directed microwave energy field to cook a food item have existed for some time. In general, a cooking process is performed by directing a standing microwave energy field into an oven cavity such that the microwave energy field reflects about the oven cavity and impinges upon the food item. As the microwave energy field impinges upon the food item, the field is converted into heat through two mechanisms. The first heating mechanism is caused by the linear acceleration of ions, generally in the form of salts present within the food item. The second is the molecular excitation of polar molecules, primarily water, present within the food item. However, the nature of the standing waves results in localized areas of high and low energy which cause the food to cook unevenly. This is especially true in larger ovens where the size of the cavity requires a more uniform energy distribution in order to properly cook the food. To attain an even or uniform energy distribution, the microwave energy must be introduced into the oven cavity in a manner which creates a constructive standing wave front which will propagate about the oven cavity in a random fashion.
Various methods of directing microwaves into cooking chambers to minimize hot and cold areas within a food item have been proposed in the prior art. These methods range from altering the pattern of the standing waves by varying the frequency of the microwave energy field, to incorporating a stationary mode stirrer which simulates a change in the geometric space of the cooking chamber. Methods of changing the wave pattern include the incorporation of a rotating blade stirrer which functions to reflect microwave energy into a cooking cavity in various patterns. Traditionally, stirrers have been located at various points in the microwave feed system, ranging from adjacent to a microwave energy source, to a position within the cooking chamber itself. Some stirrers include various openings which are provided to disperse the standing waves, while others have various surface configurations designed to reflect the standing waves. Stirrers are driven either by a motor or air currents supplied by a blower. In any case, all of these methods share a common theme, i.e., to reflect and/or deflect the microwave energy into a cooking cavity such that a more uniform distribution of standing wave patterns can be achieved.
Other methods designed to achieve a more uniform distribution include modifying the structure of the waveguide itself. Waveguide designs include cylinders, square boxes, and a variety of other configurations, each having an exit window through which the microwave energy can pass. While these designs may cause the standing waves to interfere with one another such that the wave pattern is randomized, substantial energy is typically lost with such arrangements. Still other methods are directed to rotating or moving the food being cooked within the cooking chamber. In general, the food is supported on a platter which is rotated through the standing wave patterns such that the food is more uniformly exposed to the microwaves. While these methods are fine for smaller ovens, they are hardly practical for larger ovens.
As oven cavities have grown in size and microwave technology has been combined into conventional or convection ovens, the uniform distribution of the standing waves has become of even greater concern. For this reason, manufacturers have modified their designs to include multiple magnetrons, multiple stirrers, and motor driven variable speed stirrers, all of which are intended to create a random wave pattern thought to be of a more uniform character. Certainly, the mechanisms which serve to defect the microwave energy field, e.g., stirring fans and turntables, add to the complexity of these systems and introduce multiple failure points, thus reducing the service life of such appliances. Furthermore, in an age where energy consumption is of a concern, the need for an energy efficient cooking appliance is desired.
Based on the above, there still exists a need for a microwave feed or delivery system which will direct a uniform standing wave pattern into a cooking chamber in a manner that minimizes energy losses within a waveguide, while providing a uniform, maximum energy field source to the cooking chamber.
The present invention is directed to a microwave cooking appliance including a cooking chamber and a microwave energy delivery system. The microwave energy delivery system includes an annular, toroidal-shaped waveguide, a feed member, a magnetron, and a field flux generator. More specifically, the waveguide includes an upper surface, a hollow interior portion exposed to the cooking chamber, and a circular bottom surface. The feed member serves as an interface between the magnetron and the waveguide. Preferably, the feed member is constituted by a tubular section having a first end which is open to the waveguide and a second end onto which a microwave energy source, e.g. a magnetron, is mounted.
The field flux generator operates to shift the microwave energy field in the feed member to create uniform, high energy standing microwaves to be introduced into the cooking chamber. Being that the microwave energy field includes both magnetic and electrical components, the field flux generator can generate either a magnetic field or an electrical field to flux or shift the frequency of the microwave energy field. In either case, the field flux generator is operated by one of a plurality of energy sources. More specifically, the field flux generator can be operated on a pulsed DC current, a rectified AC current or a pure AC signal. Each of the energy sources evokes a different response from the microwave energy field. Actually, each of the energy sources has a different influence on the speed at which the microwave energy field reacts such that the energy sources can be matched to particular characteristics of the cooking appliance.
Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description of a preferred embodiment when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views.